A PTAT sensing circuit is a common temperature sensing circuit applied to a situation where accurate temperature detection is needed. For example, in a global position system (GPS) device, an oscillator frequency of a local oscillator needs to be extremely precise to maintain the accuracy of positioning. However, the oscillator frequency varies with the temperature. Therefore, the GPS device also needs to accurately sense the temperature to facilitate the local oscillator to generate a proper local frequency.
Referring to FIG. 1, a conventional PTAT sensing circuit applies a pair of bipolar junction transistors (BJTs) to sense the temperature. In a PTAT sensing circuit 10, when collector current densities of transistors Q4 and Q5 are different, a collector voltage difference (ΔVEB) between the transistors Q4 and Q5 satisfies Formula 1:ΔVEB=VT ln [(IC4/A4)/(IC5/A5)]where VT is equal to kT/q, VT is a thermal voltage, k is the Boltzmann's constant, T is an absolute temperature, q is an electric charge, IC4 and IC5 are respectively collector currents of the transistors Q4 and Q5, A4 and A5 are respectively emitter areas of the transistors Q4 and Q5, and IC4/A4 and IC5/A5 are respectively current densities of the transistors Q4 and Q5. Therefore, Formula 1 shows the relationship between the emitter-collector voltage difference ΔVEB and the absolute temperature T. Furthermore, other components of the PTAT sensing circuit 10 amplify the emitter-collector voltage difference ΔVEB to generate a PTAT voltage VPTAT. VPTAT is obtained via a simple analysis:VPTAT=ΔVEB*2*(M4/M3)*(R11/R9),The following Formula 2 is obtained by substituting VPTAT into Formula 1:VPTA=VT ln [(IC4/A4)/(IC5/A5)]*2*(M4/M3)*(R11/R9),where M4/M3 is a current proportion of a current mirror formed by the transistors M3 and M4. The relationship between the PTAT voltage and the absolute temperature is thus established via Formula 2. Therefore, when the PTAT sensing circuit 10 operates, the absolute temperature being sensed is acquired according to the generated PTAT voltage.
However, a sensing error in the PTAT sensing circuit 10 may be resulted from a mismatch between its circuit components. More particularly, when the PTAT sensing circuit 10 is implemented via an integrated circuit (IC), factors during the production process of the IC inevitably cause the mismatch between the circuit components such that it is even more difficult to avoid the error. Take FIG. 1 for example. The mismatch circuit components may be the transistors Q4 and Q5, two input ends (regarded as circuit components) of an amplifier 11, the transistors M3 and M4, the resistors R8 and R10, and the resistors R9 and R11. For example, suppose that the relationships of the foregoing 5 pairs of circuit components are: an emitter area ratio of transistors Q4 and Q5 A5/A4 is 8, the amplifier 11 has no voltage offset between its two input ends, the current ratio of the current mirror formed by the transistors M3 and M4 M4/M3 is 1.5, R10/R8 is 1.5, and R11/R9 is 1.
Due to the IC manufacturing process or other factors, the foregoing relationships may become invalid, and the following circumstances are generated instead. For example, A5/A4=8*(1+ΔA4), the amplifier 11 has a voltage offset Voffset(T) between its two input ends, where the Voffset(T) changes according to the absolute temperature T, M4/M3=1.5*(1+ΔM4), R10/R8=1+ΔR8, and R11/R9=1+ΔR9. ΔA4, Voffset(T), ΔM4, ΔR8 and ΔR9 respectively represent a mismatching extent of each pair of circuit components.
Under the foregoing mismatching circumstances, a sensing error in the VPTAT obtained from Formula 2 is caused to undesirably influence the accuracy of the PTAT sensing circuit 10. Via a further experiment, it is found that the mismatch between the transistors Q4 and Q5 and between two input ends of the amplifier is a main source of the sensing error.